The field of this invention is the production of objects of material curable in response to stimulating radiation. This invention relates generally to an improved stereolithography method and system for the production of three-dimensional objects.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "Apparatus For Production of Three-Dimensional Objects By Stereolithograpy" have come into use. Basically, stereolithography is a method for automatically building complex three-dimensional objects by successively solidifying a plurality of thin layers of a solidifiable fluid-like medium by exposure to appropriate stimulation. Successive layers are solidified on top of each other until all of the thin layers are created to form a whole object (objects made in this way are sometimes called "parts"). In a preferred embodiment the fluid medium is a liquid photo-polymer that can be polymerized and solidified by exposure to UV radiation. Each polymerized layer is in essence a thin cross section of the desired three-dimensional part. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form for making prototypes. Moreover, complex parts can be made quickly without tooling. Because the system uses a computer to generate the cross section of patterns, the system can be readily linked to CAD/CAM systems.
Presently preferred polymers are cured by ultraviolet (UV) light and their curing rates are fast enough using reasonably available UV light to make them practical building materials. An ultraviolet laser generates a small intense spot of UV which is moved across the liquid surface with galvanometer or servo mirror X-Y scanners in a predetermined pattern. The scanners are driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique.
Stereolithography requires the formation of thin, uniformly-thick volumes of liquid resin which can be selectively hardened by exposure to synergistic radiation to form solid layers of the part being built. The prior art requires that the part be immersed in a vat of resin, and the formation of these resin volumes is accomplished by one of two methods, gravity recoating or blade recoating.
In gravity recoating, the part being fabricated is lowered below the resin surface to permit liquid to flow across the previously-formed layer. Generally the part is lowered by more than the thickness of the desired layer so as to accelerate flow of the viscous material; it is then raised to a position lower than its previous position by a distance equal to the desired layer thickness. The excess thickness of liquid above the previous layer is then higher than the general liquid surface, creating a bulge. After some time, the bulge flattens due to gravity and the liquid surface becomes more or less flat. This completes the formation of the volume called the "pre-layer"; it is bounded on one surface by a plane coincident with the top of the previous layer and on the opposite surface by the free working surface of the resin.
Blade recoating is similar to gravity recoating except that a blade, sweeping across the liquid surface, intercepts the bulge and trims its thickness to a dimension closer to the desired layer thickness. This is usually done while the previous layer is higher than the general liquid surface. The part is then lowered so that the previous layer is below the liquid surface by the desired layer thickness, and gravity provides the final flattening of the surface.
Both gravity and blade recoating suffer from the characteristic that as the bulge decreases in height, the forces which tend to flatten it decrease in magnitude. Therefore flattening can require a long time. In some circumstances, surface tension may even act in a way that creates an equilibrium surface which is non-flat.
The vertical resolution which can be obtained with stereolithography improves in proportion to the thinness of the layers which can be formed. Furthermore, according to the prior art, viscous resins are necessary to minimize distortions such as shrinkage and curl. Because the time required to obtain a flat working surface increases with the thinness of the pre-layer and the resin viscosity, thruput must be traded off against resolution and accuracy. Another problem is that the time required to obtain a flat working surface is a function of the pre-layer thickness and of the geometry of the layers already built, creating a variable which presently must be selected by the operator. If the previous layers are wide, flattening will be slower than if they are narrow. Trapped volumes (volumes of resin within part walls which do not communicate with the remainder of the resin except near the surface) also slow down the formation of the pre-layer.
A blade has been used to speed recoating but presents many problems. For example, the thickness of the pre-layer after the bulge has been trimmed by the blade is a function of the part geometry: it is somewhat affected by the width of regions of the previously-built part, and very sensitive to the presence of trapped volumes. To help compensate for the effects of geometry, it is possible to vary blade velocity, number of sweeps and clearance (distance between the previous layer and the blade) on a layer-by-layer basis. However, this creates even more variables and may not solve the fundamental problem of a variety of geometries on a single layer, not all of which can be optimally recoated at the same time. A blade tends to hollow-out trapped volumes, which can cause severe part deformity. Further problems with blade recoating which can create severe defects in parts include the formation of creases on the liquid surface (after use of the blade) and the formation of bubbles on the liquid surface. Finally, there are difficulties associated with the manufacture and alignment of a blade mechanism which must remain substantially parallel to the liquid surface throughout its motion.
With the prior art, curing takes place at the surface of a volume of resin. The pre-layer is in contact with the previous layer and with an underlying volume of resin. Normally the prelayer is given more exposure than is necessary to cure to a depth equal to the layer thickness; this is known as overcure. Overcure provides proper adhesion between the layer and the previous layer where the two are in contact. Additional overcure may be employed to strengthen the layer. Overcure has no effect on layer thickness in regions of the layer which are in contact with the previous layer. However, the extra exposure causes curing of the resin beneath the pre-layer in the remaining regions, with the result that these regions become thicker than the desired layer thickness. Thus to obtain proper adhesion and extra rigidity of the layer, the accuracy and uniformity of the layer thickness is sacrificed. Though it is possible to reduce the exposure of regions which do not contact the previous layer (since no adhesion is required in these), this complicates part building and still does not allow for extra rigidity.
With the prior art, the previous layers are immersed in resin during the entire part building cycle (this is because the bulk resin supplies the material and the support for the prelayers.) Therefore removal of excess resin cannot occur until the part is completely built. This forces the existence of an extra step in the production of the part, known as part stripping. Furthermore, liquid resin may be trapped by closed surfaces within volumes which are not intended to be solid (such it the case, for example, in trying to build a hollow sphere). To drain these regions the surface must be perforated and repaired afterwards. Finally, in some parts there are narrow passages and spaces which are filled with liquid resin; these must be cleaned out thoroughly before post-curing so that the part geometry is correct. Capillary adhesion and resin viscosity make it difficult to remove resin from these regions.
For further details of stereolithography, reference is made to U.S. Pat. No. 4,575,330 and the following pending U.S. patent applications which are incorporated herein by this reference in their entirety, including appendices attached thereto or material incorporated therein by reference, as if fully set forth herein:
U.S. patent application Ser. No. 339,246, filed Apr. 17, 1989, entitled "STEREOLITHOGRAPHIC CURL REDUCTION";
U.S. patent application Ser. No. 331,644, filed Mar. 31, 1989, entitled "METHOD AND APPARATUS FOR PRODUCTION OF HIGH RESOLUTION THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY";
U.S. patent application Ser. No. 183,015 now issued as U.S. Pat. No. 5,015,425, filed Apr. 18, 1988, entitled "METHOD AND APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY";
U.S. patent application Ser. No. 268,429, filed Nov. 8, 1988, entitled "METHOD FOR CURING PARTIALLY POLYMERIZED PARTS";
U.S. patent application Ser. No. 268,428, now abandoned, filed Nov. 8, 1988, entitled "METHOD FOR FINISHING PARTIALLY POLYMERIZED PARTS";
U.S. patent application Ser. No. 268,408, filed Nov. 8, 1988, entitled "METHOD FOR DRAINING PARTIALLY POLYMERIZED PARTS";
U.S patent application Ser. No. 268,816, filed Nov. 8, 1988, entitled "APPARATUS AND METHOD FOR PROFILING A BEAM";
U.S. patent application Ser. No. 268,907, filed Nov. 8, 1988, entitled "APPARATUS AND METHOD FOR CORRECTING FOR DRIFT IN PRODUCTION OF OBJECTS BY STEREOLITHOGRAPHY";
U.S. patent application Ser. No. 268,837, filed Nov. 8, 1988, entitled "APPARATUS AND METHOD FOR CALIBRATING AND NORMALIZING A STEREOLITHOGRAPHIC APPARATUS";
U.S. patent application Ser. No. 249,399, filed Sept. 26, 1988, entitled "METHOD AND APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY";
U.S. patent application Ser. No. 365,444, filed June 12, 1989, entitled "INTEGRATED STEREOLITHOGRAPHY"; and
U.S. patent application Ser. No. 265,039, now abandoned, filed Oct. 31, 1988, entitled "APPARATUS AND METHOD FOR MEASURING AND CONTROLLING THE LEVEL OF A FLUID".
U.S. patent application Ser. No. 269,801, now abandoned, filed Mar. 31, 1989, entitled "METHOD AND APPARATUS FOR PRODUCTION OF HIGH RESOLUTION THREE DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY".